In general, spark plugs include a center electrode and a ground electrode in a front end portion thereof. To date, in order to address a need for improvement of ignitability and wear resistance of spark plugs, spark plugs having a noble metal tip that is joined to a part of the ground electrode near one end of the ground electrode have been used.
In general, the coefficient of thermal expansion differs between the noble metal tip and the ground electrode. Therefore, when the spark plug is subjected to thermal cycles during use, the noble metal tip may become separated from the ground electrode. For this reason, to date, various joining methods have been devised to prevent separation of the noble metal tip (see Japanese Unexamined Patent Application Publication No. 2005-123182 and Japanese Unexamined Patent Application Publication No. 2012-074271).
However, because spark plugs are more likely to be used under severer conditions in recent years, further improvement in the separation resistance of the noble metal tip is needed.
The present invention, which has been devised to solve the aforementioned problem.